2007B_phys208_Exam4A_solutions - Version A - Page 9 Problem...

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Unformatted text preview: Version A - Page 9 Problem 1 (20 points): A small plastic ball (mass m = 1.5 g) is tied to a very light 2.00 cm string that is attached to the ceiling. See the figure. A uniform horizontal electric field exists between two plates. When the ball has been given an excess charge of q = 147 µC, you observe that it remains suspended, with the string making an angle of θ = 30.0o with the vertical. Find the magnitude and direction of electric field. Version A - Page 10 Problem 2 (20 points): Your assignment in a PHYS 208 lab is to characterize the motion of a rectangular circuit shown in the figure. The rectangular loop is hinged along the y axis. It carries a 15.0 A current and is located in a uniform 1.20 T magnetic field oriented in the +x direction. (a) (10 pts) Make a clear sketch showing the direction of the force that the magnetic field exerts on each segment of the circuit. (b) (10 pts) Of four forces you drew in part (a), decide which one(s) exerts a torque about the hinge. Then calculate the magnitude of only the force(s) that exerts this torque. (1) (2) (3) (4) 2 pts 2 pts 4 pts 2 pts Version A - Page 11 Problem 3 (20 points): In a PHYS 202 lab, you are assigned to calculate the time response of the circuit ( ε = 100 V, R1 = 10 , R2 = 30 , and L = 2.0 H) before its measurements. See the figure. Find (a) (10 pts) the current through each of the resistor just after the switch (S) is closed; (b) (5 pts) the current through each resistor after S has been closed a long time; (c) (5 ps) the current through R1 right after S is re-opened in part (b). Version A - Page 12 Problem 4 (20 points): You are a chief engineer. General Electric at Texas Corp (GET Corp.) just hires Dr. Kamon as a chief engineer. His first task is to evaluate your design of metal detector. You explain using the figure: In its simplest form, a metal detector consists of an oscillator producing a varying current I0(t) that passes through a coil r (transmitting coil) producing a varying magnetic field B0 to the right. If an electrically-conductive metal plate is close to the coil, eddy currents will be r induced in the metal, and this produces an varying magnetic field B of its r own to the right. If another coil (receiver coil) is used to measure B , the r change in B will be detected as a varying current I in the receiver coil. He asks: (a) (5 pts) What is the principal physics behind this technology? (b) (5 pts) Sketch the direction of the eddy current on the metal plate, r assuming the direction of B in the figure is correct. (c) (10 pts) Is I0(t) increasing or decreasing to observe I as in the figure? Or both? Why? Version A - Page 13 Problem 5 (10 points): EM Wave A small helium-neon laser emits red visible light with a pow...